1. Field of the Invention
The present invention relates to a fuel control apparatus for an engine and, more particularly, to a fuel control apparatus adapted to carry out a control by learning, in addition to a feedback control.
2. Description of the Prior Art
In an engine, particularly an internal combustion engine for vehicles, an air-fuel ratio is frequently controlled according to an output from an air-fuel ratio sensor such as an O.sub.2 sensor, that is, a fuel amount supplied to the engine is frequently controlled (or corrected) so that the air-fuel ratio of a mixture gas become a target value.
This feedback control has a problem in the responsiveness of the control. Thus, control by learning or a learning control, in addition to the feedback control, has been recently proposed. In Japanese Patent Application Laid Open No. 59335/1983, the feedback correction is conducted using a feedback correction value that is obtained in accordance with an output from an O.sub.2 sensor for detecting the oxygen concentration (air-fuel ratio) in exhaust gas. A learning value is calculated according to the feedback correction value and the learning value is stored in memory means having, for example, a plurality of learning zones divided at every predetermined vehicle speed. At a certain vehicle speed while conducting a sort of prospect control by the learning correction in accordance with the learning value stored in the learning zone of the memory means, corresponding to the vehicle speed, the feedback control as described hereinabove is carried out. Accordingly, an amount of correction by the feedback control (feedback correction value) can be reduced by the amount of the prospect control with the learning value, thus leading to a higher responsiveness of the control.
In particular, according as an increase in the number of learnings as the same driving state is continued for a long period of time, the amount of correction by the feedback correction can be extremely reduced. Also such a learning control may absorb the individual difference of engines, in particular, the individual difference of fuel injection valves, which affects the setting of supplying the fuel amount to a great extent or the individual difference of sensors for detecting the amount of intake air.
However, in the conventional learning control, a problem arises in the control response in instances where a learning value used for the learning correction is altered, leading to a lack in the control accuracy of an air-fuel ratio. More specifically, if the memory means for storing the learning value has a plurality of learning zones divided at every predetermined vehicle speed as described hereinabove, it is common that the learning values stored in two learning zones are different when one learning zone is altered to the other learning zone. Accordingly, if the learning value altered is used immediately after the alteration of the learning zone the control is caused to deviate because the feedback correction value is set on the basis of the learning value before alteration. Accordingly when the learning value to be used for the learning correction is altered, the feedback correction value was heretofore set to a value that is not attributable to any learning value, i.e., initialized to "zero". With such an intialization of the feedback correction value, it requires a considerable amount of time until the air-fuel ratio is stabilized to the target air-fuel ratio after the learning zone is changed.
The delay of the control caused by the alteration of the learning value as described above occurs not only in the case of altering the learning zone but in the case of renewing the learning value, i.e., in the case of updating to the optimum learning value.